The invention relates generally to heat sinks for electronic devices, and more specifically, to a foil gap filler to reduce thermal resistance between heat sinks and a microprocessors and other integrated circuit (IC) devices.
It is well known to use a heat sink to cool a heat generating IC device or package. Typically, a heat sink is arranged in close contact with a heat generating IC package, such as a Central Processing Unit (CPU). Heat is removed from the CPU to prevent the CPU from shutting down to avoid thermal overload. Heat generated by the CPU is transferred to the heat sink and then dissipated from the heat sink to the surrounding air. To further facilitate the dissipation of heat from the IC package a fan can be used to circulate air about outer surfaces of the heat sink.
Heat is transferred from the IC package to the heat sink most effectively by direct surface contact between the interfacing surface of the IC package, such as the case or upper lid of the CPU, and the base of the heat sink. However, the surfaces of the heat sink and IC package cannot be made completely even or smooth. That is, there is always some unevenness in the material surfaces such that an air gap will exist between the two abutted surfaces of the IC package and the heat sink. Since air has a relatively poor thermal conductivity as compared to most metals and other materials typically used in the manufacture of heat sinks and the outer cases of the IC packages, most heat sink manufacturers polish the heat sink interface in an effort to minimize the air gap. If the air gap could be eliminated, the transfer of heat from the IC package to the heat sink could be enhanced.
Historically, in some electronic systems, such as, for instance, a pin grid array (PGA) system, where a PGA package, such as a PGA microprocessor, is loaded onto a PGA socket on a motherboard, a liquid or phase change thermal interface material (TIM) has been used to enhance heat transfer from the PGA package to the heat sink. Typically, the TIM is a silicon grease loaded with aluminum and zinc or a wax based phase change material. When the IC package heats up, the TIM liquefies to fill the air gaps between the heat sink and the IC package and enhance the transfer of heat from the IC package to the heat sink.
In the more recently developed land grid array (LGA) system, the LGA socket has a field of spring contacts in the base of the socket. The base of the LGA package has a complementary field of contact pads that touch the spring contacts when the LGA package is compressively loaded onto the LGA socket. The phase change and loaded silicone TIM's are generally not suited for use in the package-to-heat sink interface in LGA applications. If used with an LGA system, the liquefied TIM could migrate off of the LGA package and into the spring contacts in the LGA socket causing intermittent connections between the spring contacts and the contact pads on the LGA package and interfering with the operation of the LGA package. Such problems tend to not be experienced with PGA systems since the pins on the PGA package fit through a plastic screen on the top of the PGA socket. By contrast, the spring contacts in the LGA socket and the contact pads on the base of the LGA package are both exposed.